Sorting Data in a Dataset in ASP.NET

I have a custom class that takes LoginID, FirstName, Lastname as params and does the following.
1) Query Active Directory for any people that match any of the three values above.
2) Plug the search results into a dataset and return this dataset object.

In my ASPX page, I have a repeater control and three textfields to allow a user to enter a firstname, lastname, login ID that is passed to the custom class. When the user clicks the button, the code-behind code calls the custom class, performs the search and returns a dataset. I want to be able to sort this data, but the code I am using does not seem to work. Am I doing something wrong in the code?